As it is known, materials reduced to a nanometric scale show a modification of the physical-chemical properties and functionality thereof. Among the other functional properties, the optical absorption of the metal nanoparticles or nanostructures is affected by the excitation of localized surface plasmon resonances (LSPRs) of the conduction electrons excited by the electromagnetic field of light. By modifying the nanostructure shape and symmetry, it is possible to adjust the surface plasmon frequency. In the proximity of the metal nanostructures, due to their small curvature radius, it is further noticed a strong amplification of the field [C. F. Bohren and D. R. Huffman, Absorption and Scattering of light by Small Particles, New York: John Wiley and Sons, Inc. (1983)].
The production of arrays of finely spaced metal nanostructures capable of supporting plasmon resonances is of interest in applications where high field amplification effects are required, such as second harmonic generation [H. J. Simon, D. E. Mitchell, J. G. Watson, Phys. Rev. Lett. 7, 385 (1985)], surface-enhanced Raman scattering [S. M. Nie, S. R. Emery, Science 275, 1102 (1997)] or fluorescence amplification by metal nanostructures [P. Bharadwaj, P. Anger, L. Novotny, Nanotechnology 18, 044017 (2007)].
Furthermore, the production of arrays of finely spaced metal nanostructures on transparent dielectric substrates capable of supporting plasmon resonances is of interest in advanced photovoltaic applications. It has been shown that the introduction of metal nanostructures capable of supporting plasmon resonances at the interface of a photovoltaic element p-n junction involves the amplification of the conversion efficiency [D. M. Schaadt, B. Feng, and E. T. Yu, Applied Physics Letters 86, 063106 (2005)].
Processes for the formation of metal nanowires and nanorods by controlled chemical synthesis are well documented in the scientific literature. The nanostructures show optical properties which are adjustable by modifying their plasmon absorption [B. J. Wiley, S. H. Im, Z. Li, J. McLellan, A. Siekkinen, Y. Xia, J. Phys. Chem. B 110, 15666-15675 (2006)].
In order to exploit their optical/plasmon properties, the metal nanowires have to be supported on a substrate, while maintaining the morphological, structural, and chemical integrity thereof. This is a critical aspect when using approaches of a chemical type [Catherine J. Murphy, Tapan K. Sau, Anand Gole, and Christopher J. Orendorff, MRS Bulletin 30, 349 (2005)]. There are significant problems related to the fixing of the nanostructures on a substrate, the alignment thereof, and the formation of nanowire arrays with a controllable lateral separation [Anand Gole, Christopher J. Orendorff, and Catherine J. Murphy, Langmuir 2004, 20, 7117-7122]. Further drawbacks of the chemical-type approaches relate to the use of toxic chemicals.
Instead, the physical-type approaches to the synthesis of nanowire arrays based on templates (pre-structured substrates) manufactured by lithography (for example, focused electron beam or ion beam lithography) have a low throughput. In fact, the lithographic processes involve a long sequence of processing steps. There are also technical limitations regarding the substrates which can be employed in such processes, when considering the constraints given by thermal stability, vacuum, and chemistry of the substrate and film.
The physical approaches of the self-organized (or self-ordered) type are actively studied as a low cost alternative for the large scale production and on large areas of arrays of dimensionally selected and laterally ordered metal nanostructures.
The present invention develops in the field of such alternative approaches based on physical synthesis, which involve the use of equipment generally used in the microelectronics and coating (thin film depositions) industry, and particularly the use of ion beams under vacuum conditions.
In an example, [Thomas W. H. Oates, Adrian Keller, Stefan Facsko, Arndt Mücklich, Plasmonics 2, 47 (2007)] a ion beam process is employed to manufacture a nanostructured Si template (with nanoscale corrugations). The successive deposition of Au on the template, followed by high temperature annealing, causes the agglomeration of Au into nanometric scale approximately spherical aggregates which are laterally aligned in the Si valleys. The array of aligned nanoparticles shows weakly anisotropic optical and plasmon properties. Such approach, however, cannot be applied to substrates susceptible to damage by ion irradiation or thermal annealing.
In a second example, [E. Fort, C. Ricolleau, J. Sau-Pueyo, Nanoletters, 3, 65-67 (2003)], Ag nanoparticles are arranged by forming chains in the valleys of an alumina faceted template, by thermally-induced agglomeration. The anisotropic distribution of the particles involves a dichroic optical absorbance. Such approach has a limited use, since it cannot be applied to substrates susceptible to damage by the extreme thermal annealing cycles necessary for the production of the template; furthermore, the choice of the materials exhibiting a regular faceting is limited (substantially, they are those substrates corresponding to the polar termini of crystalline materials with ionic linkage).
In a third example, [T. Kitahara, A. Sugawara, H. Sano, G. Mizutani, J. Applied Physics 95, 5002 (2004)] Au nanowires are prepared by shadow deposition (grazing-angle evaporation) on faceted NaCl substrates. The Au nanowires induce a polarized excitation of second-harmonic signals. Said approach also finds a limited use, since it cannot be applied to substrates susceptible to damage by the high temperature annealing cycles necessary for the production of the template; furthermore, the choice of the materials exhibiting a regular faceting is limited (see above).
The ion irradiation-induced self-organization processes have been the subject matter of investigation by the inventors. It is known that ion irradiation induces a morphological instability also on monocrystalline metal substrates. Recently, the inventors demonstrated that it is possible to obtain nanometric scale periodic patterns on the surface of monocrystalline metals grown in a self-organized manner by controlled irradiation with a defocused beam of noble gas ions, according to a technique hereinafter referred to as ion beam sputtering (IBS) [S. Rusponi, C. Boragno, and U. Valbusa, Phys. Rev. Lett. 78, 2795 (1997)] [S. Rusponi, G. Costantini, F. Buatier de Mongeot, C. Boragno, U. Valbusa, Appl. Phys. Lett. 75, 3318 (1999); U. Valbusa, C. Boragno and F. Buatier de Mongeot, J. Phys.: Condens. Matter 14 (2002) 8153-8175]. In brief, a defocused ion beam destabilizes an otherwise planar surface, dislocating mobile adatoms and vacancies, which rearrange by thermally-activated diffusion, producing an array of periodic nanostructures (corrugations, pyramids) which choose a preferential periodicity (in the range of 10 nm-200 nm). The periodicity and the orientation of the nanostructures is determined by the competition of a levelling term (thermally-activated diffusion) and a corrugating term, due to the erosive action of the ion beam. By acting on energy of the ions, ion flow, ion dose, incidence angle, and temperature of the substrate, it is possible to modify the morphological parameters of the nanostructures (lateral periodicity, slope of the facets, orientation) [R. M. Bradley and J. M. Harper, J. Vac. Sci. Technol. A 6, 2390 (1988)].
Furthermore, the inventors reported the possibility to extend the IBS to epitaxial crystalline films of ferromagnetic material (Co or Fe) supported on a monocrystalline substrate [R. Moroni, D. Sekiba, F. Buatier de Mongeot, G. Gonella, C. Boragno, L. Mattera, U. Valbusa Physical Review Letters 91, 167207 (2003); F. Bisio, R. Moroni, F. Buatier de Mongeot, M. Canepa and L. Mattera, Physical Review Letters 96, 057204 (2006)]. In these examples, the planar interface of the ferromagnetic film is modified by IBS into a corrugated surface. The magnetic anisotropy of the nanostructured film is obtained in accordance with the morphology of the nanostructured film.
Therefore, object of the present invention is to implement a method for the synthesis of an array of metal nanowires adapted to support localized plasmon resonances, which requires the use of general use and low cost pieces of equipment, and which allows a high yield on large areas, having the feature to allow using low cost support dielectric substrates such as amorphous materials (glasses) or polymeric films.